The previous post explored why E2 elimination reactions occur with an antiperiplanar geometry for the transition state. Here I have tweaked the initial reactant to make the overall reaction exothermic rather than endothermic as it was before. The change is startling.
The exothermicity is of course due to the aromatisation of the ring. The IRC is however quite different from before.
So, like the SN1 reaction discussed in another post, this E2 reaction occurs in distinct stages, the first resembling an E1 mechanism, followed by a second phase leading to elimination. It is still a concerted reaction, but the proton transfer occurs only AFTER the transition state. The simple designation E2/E1 is clearly not a fully adequate description of such mechanisms.
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